Discrete Polymer in Fan-Out Packages

ABSTRACT

A package includes a first molding material, a lower-level device die in the first molding material, a dielectric layer over the lower-level device die and the first molding material, and a plurality of redistribution lines extending into the first dielectric layer to electrically couple to the lower-level device die. The package further includes an upper-level device die over the dielectric layer, and a second molding material molding the upper-level device die therein. A bottom surface of a portion of the second molding material contacts a top surface of the first molding material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No. 14/690,081, entitled “Discrete Polymer in Fan-Out Packages,” filed on Apr. 17, 2015, which application is incorporated herein by reference.

BACKGROUND

With the evolving of semiconductor technologies, semiconductor dies are becoming increasingly smaller. More functions, however, need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of Input/output (I/O) pads packaged into smaller areas, and the density of the I/O pads rises quickly. As a result, the packaging of the semiconductor dies becomes more difficult, and adversely affecting the yield.

Package technologies can be divided into two categories. One category is typically referred to as Wafer Level Package (WLP), wherein the dies on a wafer are packaged before they are sawed. The WLP technology has some advantageous features, such as a greater throughput and a lower cost. Further, less underfill or molding compound is needed. However, the WLP technology suffers from drawbacks. The conventional WLP can only be fan-in type packages, in which the I/O pads of each die are limited to a region directly over the surface of the respective die. With the limited areas of the dies, the number of the I/O pads is limited due to the limitation of the pitch of the I/O pads. If the pitch of the pads is to be decreased, solder bridging may occur. Additionally, under the fixed ball-size requirement, solder balls must have a certain size, which in turn limits the number of solder balls that can be packed on the surface of a die.

In the other category of packaging, dies are sawed from wafers before they are packaged onto other wafers, and only “known-good-dies” are packaged. An advantageous feature of this packaging technology is the possibility of forming fan-out packages, which means the I/O pads on a die can be redistributed to a greater area than the die itself, and hence the number of I/O pads packed on the surfaces of the dies can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking fan-out package in accordance with some embodiments;

FIG. 12 illustrates a top view of a portion of the multi-stacking fan-out package;

FIG. 13 illustrates a magnified view of a portion of a multi-stacking fan-out package in accordance with some embodiments;

FIGS. 14 through 16 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking fan-out package in accordance with alternative embodiments, wherein no opening is formed in upper polymer layers;

FIGS. 17 and 18 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking fan-out package in accordance with alternative embodiments, wherein a liquid molding compound is applied;

FIGS. 19 through 21 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking fan-out package in accordance with alternative embodiments, wherein a partial cut is performed prior to applying a liquid molding compound; and

FIG. 22 illustrates a process flow for forming a multi-stacking fan-out package in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

A multi-stacking fan-out packages and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages of forming the multi-stacking fan-out packages are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking fan-out package in accordance with some embodiments of the present disclosure. In the subsequent discussion, the process steps shown in FIGS. 1 through 11 are discussed referring to the process steps in FIG. 12.

Referring to FIG. 1, carrier 30 is provided, and adhesive layer 32 is disposed over carrier 30. Carrier 30 may be a blank glass carrier, a blank ceramic carrier, or the like, and may have a shape of a semiconductor wafer with a round top-view shape. Carrier 30 is sometimes referred to as a carrier wafer. Adhesive layer 32 may be formed of a Light-to-Heat Conversion (LTHC) material, for example, and other types of adhesives may also be used. In accordance with some embodiments of the present disclosure, adhesive layer 32 is capable of decomposing under the heat of light, and hence can release carrier 30 from the structure formed thereon.

Buffer layer 34 is formed over adhesive layer 32. In accordance with some embodiments of the present disclosure, buffer layer 34 is a polymer layer formed of polybenzoxazole (PBO), polyimide, or the like.

Device dies 36 are placed over buffer layer 34. The respective step is shown as step 202 in the process flow shown in FIG. 22. Device dies 36 may be adhered to buffer layer 34 through die attach films 38. The edges of die attach films 38 are co-terminus with (aligned to) the edges of the respective overlaying device dies 36. Die attach films 38 are adhesive films. The plurality of placed device dies 36 may be arranged as an array including a plurality of rows and a plurality of columns. Device dies 36 may include a semiconductor substrate having a back surface (the surface facing down) in contact with the respective die attach films 38. Device dies 36 further include integrated circuit devices (such as active devices, which include transistors, for example, not shown) at the front surfaces (the surface facing up) of the semiconductor substrates. Device dies 36 may include memory dies such as Static Random Access Memory (SRAM) dies, Dynamic Random Access Memory (DRAM) dies, or the like.

Device dies 36 include conductive features such as metal pillars 40 at their top surfaces. Metal pillars 40 are electrically coupled to the integrated circuits inside device dies 36. In accordance with some exemplary embodiments of the present disclosure, as shown in FIG. 1, the top surfaces of metal pillars 40 are exposed. Metal pillars 40 may be copper pillars, and may also include other conductive/metallic materials such as aluminum, nickel, or the like. In accordance with some embodiments of the present disclosure, the top surfaces of metal pillars 40 are coplanar with the top surface of dielectric layer 42. In accordance with alternative embodiments of the present disclosure, metal pillars 40 are embedded in dielectric layer 42, with the top surfaces of dielectric layers 42 being higher than the top surfaces of metal pillars 40. Dielectric layer 42 may be formed of a polymer, which may include PBO, polyimide, or the like. Throughout the description, device dies 36 are also referred to as level-1 dies or lower-level dies.

Referring to FIG. 2, molding material 44 is molded on device dies 36. The respective step is shown as step 204 in the process flow shown in FIG. 22. Molding material 44 is dispensed as a fluid and is then cured, for example, in a thermal curing process. Molding material 44 fills the gaps between device dies 36 and may be in contact with buffer layer 34. Molding material 44 may include a molding compound, a molding underfill, an epoxy, or a resin. After the molding process, the top surface of molding material 44 is higher than the top ends of metal pillars 40.

Next, a planarization step such as a Chemical Mechanical Polish (CMP) step or a grinding step is performed to planarize molding material 44, until metal pillars 40 of device dies 36 are exposed. The respective step is shown as step 206 in the process flow shown in FIG. 22. The resulting structure is shown in FIG. 3. Due to the planarization, the top surfaces of metal pillars 40 are substantially level (coplanar) with the top surface of molding material 44.

Referring to FIG. 4, one or more layers of dielectric layers 46 and the respective Redistribution Lines (RDLs) 48 are formed over molding material 44 and metal pillars 40. The respective step is shown as step 208 in the process flow shown in FIG. 22. In accordance with some embodiments of the present disclosure, dielectric layers 46 are formed of a polymer(s) such as PBO, polyimide, or the like. In accordance with alternative embodiments of the present disclosure, dielectric layers 46 are formed of an inorganic dielectric material(s) such as silicon nitride, silicon oxide, silicon oxynitride, or the like.

RDLs 48 are formed to electrically couple to metal pillars 40. RDLs 48 may include metal traces (metal lines) and vias underlying and connected to the respective metal traces. In accordance with some embodiments of the present disclosure, RDLs 48 are formed through plating processes, wherein each of RDLs 48 includes a seed layer (not shown) and a plated metallic material over the seed layer. The seed layer and the plated metallic material may be formed of the same material or different materials.

During the formation of RDLs 48, dielectric layers 46 are patterned to form via openings (occupied by RDLs 48), and upper-level RDLs 48 extend into the via openings to contact lower-level RDLs 48 or metal pillars 40. At the same time the via openings are formed, some portions of dielectric layers 46 are also removed to form openings 49 in dielectric layers 46. The formation of via openings and openings 49 may be performed using a same lithography mask and same lithography processes. Openings 49 may form a grid including horizontal openings (trenches, viewed from top) crossing vertical openings. The remaining portions of dielectric layers 46 are thus in the regions defined by the grid.

Referring to FIG. 5, metal posts 50 are formed over dielectric layers 46, and are electrically connected to RDLs 48. Throughout the description, metal posts 50 are alternatively referred to as through-vias 50 since metal posts 50 penetrate through the subsequently formed molding material. The respective step is shown as step 210 in the process flow shown in FIG. 22. Through-vias 50 are connected to RDLs 48 through the vias formed in the top layer of dielectric layers 46. In accordance with some embodiments of the present disclosure, through-vias 50 are formed by plating, and the formation process may include patterning the top layer of dielectric layers 46 to form openings to expose RDLs 48, forming a blanket seed layer (not shown) extending into the openings, forming and patterning a photo resist (not shown), and plating through-vias 50 on the portions of the seed layer that are exposed through the openings in the photo resist. The photo resist and the portions of the seed layer that were covered by the removed photo resist are then removed. The material of through-vias 50 may include copper, aluminum, nickel, tungsten, or the like. Through-vias 50 have the shape of rods. The top-view shapes of through-vias 50 may be circles, rectangles, squares, hexagons, or the like.

FIG. 6 illustrates the placement of device dies 52 over dielectric layers 46. Throughout the description, device dies 52 are also referred to as level-2 dies or upper-level dies. The respective step is shown as step 212 in the process flow shown in FIG. 22. Device dies 52 may be adhered to the top dielectric layer 46 through die attach films 54, which are adhesive films. Each of device dies 52 may include a semiconductor substrate having a back surface (the surface facing down) in physical contact with the respective die attach film 54. Device dies 52 further includes integrated circuit devices (such as active devices, which include transistors, for example, not shown) at the front surface (the surface facing up) of the semiconductor substrate. Device dies 52 may include logic dies such as Central Processing Unit (CPU) dies, Graphic Processing Unit (GPU) dies, mobile application dies, or the like. Device dies 52 may also include System on Chip (SoC) dies.

Device dies 52 include conductive features such as metal pillars 56 at their top surfaces. Metal pillars 56 are electrically coupled to the integrated circuits inside device dies 52. In accordance with some exemplary embodiments of the present disclosure, as shown in FIG. 6, the top surfaces of metal pillars 56 are exposed. Metal pillars 56 may be copper pillars, and may also include other conductive/metallic materials such as aluminum, nickel, or the like. In accordance with some embodiments of the present disclosure, the top surfaces of metal pillars 56 are coplanar with the top surface of dielectric layer 58. In accordance with alternative embodiments of the present disclosure, metal pillars 56 are embedded in dielectric layer 58, with the top surfaces of dielectric layers 58 being higher than the top surfaces of metal pillars 56. Dielectric layer 58 may be formed of a polymer, which may include PBO, polyimide, or the like.

Referring to FIG. 7, molding material 60 is molded on device dies 52 and through-vias 50. The respective step is shown as step 214 in the process flow shown in FIG. 22. Molding material 60 may include a molding compound, a molding underfill, an epoxy, or a resin. After the molding process, the top surface of molding material 60 is higher than the top ends of metal pillars 56 and through-vias 50. Molding material 60 is filled into openings 49 (FIG. 6) to contact molding material 44. In accordance with some embodiments of the present disclosure, molding material 44 and molding material 60 are formed of a same type of molding material. In accordance with alternative embodiments, molding material 44 and molding material 60 are formed of different types of molding materials.

Next, a planarization step is performed to planarize molding material 60, until through-vias 50 are exposed. The respective step is shown as step 216 in the process flow shown in FIG. 22. The resulting structure is shown in FIG. 8. Metal pillars 56 of device dies 52 are also exposed as a result of the planarization. Due to the planarization, the top surfaces of through-vias 50 are substantially level (coplanar) with the top surfaces of metal pillars 56, and are substantially level (coplanar) with the top surface of molding material 60.

Referring to FIG. 9, one or more layers of dielectric layers 62 and the respective RDLs 64 are formed over molding material 60, through-vias 50, and metal pillars 56. The respective step is shown as step 218 in the process flow shown in FIG. 22. In accordance with some embodiments of the present disclosure, dielectric layers 62 are formed of a polymer(s) such as PBO, polyimide, or the like. In accordance with alternative embodiments of the present disclosure, dielectric layers 62 are formed of an inorganic dielectric material(s) such as silicon nitride, silicon oxide, silicon oxynitride, or the like.

RDLs 64 are formed to electrically couple to metal pillars 56 and through-vias 50. RDLs 64 may also electrically interconnect metal pillars 56, through-vias 50, and metal pillars 40 with each other. RDLs 64 may include metal traces (metal lines) and vias underlying and connected to the metal traces. In accordance with some embodiments of the present disclosure, RDLs 64 are formed through plating processes, wherein each of RDLs 64 includes a seed layer (not shown) and a plated metallic material over the seed layer. The seed layer and the plated metallic material may be formed of the same material or different materials.

During the formation of RDLs 64, dielectric layers 62 are also patterned to form via openings, and upper-level RDLs 64 extend into the via openings to contact lower-level RDLs 64 or metal pillars 56. At the same time the via openings are formed, some portions of dielectric layers 62 are also removed to form openings 66 in dielectric layers 62. The formation of via openings and openings 66 may be performed using a same lithography mask and same lithography processes. Openings 66 may form a grid, with the remaining portions of dielectric layers 62 being in the regions defined by the grid. Molding material 60 is thus exposed through openings 66.

FIG. 10 illustrates the formation of electrical connectors 68 in accordance with some exemplary embodiments of the present disclosure. The respective step is shown as step 224 in the process flow shown in FIG. 22. Electrical connectors 68 are electrically coupled to RDLs 64, metal pillars 40 and 56, and/or through-vias 50. The formation of electrical connectors 68 may include placing solder balls over RDLs 64 and then reflowing the solder balls. In accordance with alternative embodiments of the present disclosure, the formation of electrical connectors 68 includes performing a plating step to form solder regions over RDLs 64 and then reflowing the solder regions. Electrical connectors 68 may also include metal pillars, or metal pillars and solder caps, which may also be formed through plating. Throughout the description, the combined structure including device dies 52, through-vias 50, molding material 60, RDLs 64, and dielectric layers 62 will be referred to as package 70, which is also a composite wafer.

In accordance with some embodiments of the present disclosure, width W1 of opening 49 (also refer to FIG. 4) is in the range between about 100 μm and about 300 μm. Width W2 of openings 66 is in the range between about 100 μm and about 300 μm. Width W2 may be greater than or equal to width W1. Width W3 of the saw-line is in the range between about 30 μm and about 40 μm.

Next, package 70 is de-bonded from carrier 30. In accordance with some exemplary de-boding process, a dicing tape (not shown) is attached to package 72 to protect electrical connectors 68. The de-bonding of package 70 from carrier 30 is performed, for example, by projecting a UV light or a laser on adhesive layer 32. For example, when adhesive layer 32 is formed of LTHC, the heat generated from the light or laser causes the LTHC to be decomposed, and hence carrier 30 is detached from package 72. In a subsequent step, die-saw is performed to saw package 70 into discrete packages 72. The respective step is also shown as step 224 in the process flow shown in FIG. 22.

FIG. 11 illustrates a resulting discrete package 72. Package 72 includes device dies 36 at a lower level, and device dies 52 at an upper level. Device dies 36 and 52 are electrically interconnected through RDLs 48 and 64 and through-vias 50 to form a multi-stacking package 72. Dielectric layers 46, which may be polymer layers, are formed between lower-level die(s) 36 and upper-level dies 52. Lower-level dies 36 are molded in molding material 44, and upper-level dies 52 are molded in molding material 60. Molding material 60 extends down, with a portion of molding material 60 being coplanar with dielectric layers 46.

FIG. 12 illustrates a top view of a portion of discrete package 72, wherein the top view is obtained at the level of dielectric layers 46. As shown in FIG. 12, a portion of molding material 60 (referred to as a ring portion hereinafter) encircles dielectric layers 46. Also, edges 62A of dielectric layers 62 are misaligned with the respective edges of molding materials 44 and 60, and are recessed more toward the center of package 72.

In accordance with some embodiments of the present disclosure, edge 62A of dielectric layers 62 is recessed from edge 60A of molding material 60 (and edge 44A of molding material 44) by recessing distance D1, which may be in the range between about 30 μm and about 130 μm. The width of the ring portion of molding material 60 is equal to recessing distance D2, which is the recessing distance of edge 46A of dielectric layers 46 recessed from edge 60A of molding material 60. Recessing distance D2 may also be in the range between about 30 μm and about 130 μm.

Furthermore, the bottom surface of the ring portion of molding material 60 is coplanar with the bottom surface of dielectric layers 46, as shown in FIG. 11, and is in contact with the top surface of molding material 44 to form interface 80. Regardless of whether molding materials 44 and 60 are formed of a same material or different molding materials, interface 80 is distinguishable due to the planarization of molding material 44. For example, FIG. 13 illustrates a schematic magnified view of a portion of package 72 in region 74 (FIG. 11). Molding material 44 may include fillers 76A in polymer 78A. Molding material 60 may include fillers 76B in polymer 78B. Fillers 76A and 76B may be formed of silica, aluminum oxide, aluminum nitride, silicon carbide, or the like, and the material of fillers 76A may be the same or different from the material of fillers 76B. Furthermore, fillers 76A and 76B may have spherical shapes. Due to the planarization of molding material 44, fillers 76A are also grinded, and the top portions of some fillers 76A are removed. Accordingly, some top portions of fillers 76A may have planar top surfaces coplanar with interface 80, with these fillers 76A having round lower parts. On the other hand, some fillers 76B that contact interface 80 have rounded bottom ends coplanar with interface 80. Accordingly, interface 80 is distinguishable through the shapes of fillers 76A and 76B.

FIGS. 14 through 16 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking package in accordance with alternative embodiments of the present disclosure. Unless specified otherwise, the materials and the formation methods of the components in these embodiments are essentially the same as the like components, which are denoted by like reference numerals in the embodiments shown in FIGS. 1 through 13. The details regarding the formation process and the materials of the components shown in FIGS. 14 through 16 (and also FIGS. 17 through 21) may thus be found in the discussion of the embodiments shown in FIGS. 1 through 13.

The initial steps of these embodiments are essentially the same as shown in FIGS. 1 through 8. Next, as shown in FIG. 14, dielectric layers 62 and RDLs 64 are formed. The formation process is similar to the process in FIG. 9, except that when via openings are formed in dielectric layers 62, no openings are formed between device dies 52. Alternatively stated, openings 66 as shown in FIG. 9 are not formed in accordance with these embodiments. Next, as shown in FIG. 15, electrical connectors 68 are formed, and a die-saw is performed to separate package 70 into packages 72. A resulting discrete package 72 is shown in FIG. 16. The package 72 in FIG. 16 is similar to the package 72 shown in FIG. 11, except that the edges of dielectric layers 62 are vertically aligned to (co-terminus with and it the same vertical plane as) the respective edges of molding materials 44 and 60.

FIGS. 17 and 18 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking package in accordance with yet alternative embodiments of the present disclosure. The initial steps of these embodiments are essentially the same as shown in FIGS. 1 through 9. Next, as shown in FIG. 17, liquid molding compound 74 is applied to protect package 70, wherein the lower parts of electrical connectors 68 are molded in liquid molding compound 74. The respective step is shown as step 222 in the process flow shown in FIG. 22, wherein the dashed box indicates step 222 may be performed or skipped in different embodiments. The molding process includes applying liquid molding compound 74, and pressing liquid molding compound 74 using a release film (not shown), so that the top parts of electrical connectors 68 are pressed into the release film, and the excess liquid molding compound 74 is squeezed away from package 72 by the release film. Liquid molding compound 74 is then cured. The release film is then removed, leaving the structure in FIG. 17. The cured liquid molding compound 74 is referred to as molding material 74 hereinafter. Molding materials 60 and 74 may be formed of a same molding material, or may be formed of different molding materials.

Package 70 is then de-bonded from carrier 30, and is sawed. The resulting discrete package 72 is shown in FIG. 18. In package 72, in addition to the ring portion (a full ring) of molding material 60 that encircles dielectric layers 46, molding material 74 also has a ring portion (a full ring) encircling dielectric layers 62. The bottom surface of molding material 74 contacts the top surface of molding material 60 to form interface 76. Interface 76 is distinguishable since molding material 60 is grinded or polished, and hence the fillers in molding material 60 will also be polished to have planar top surfaces coplanar with interface 76. The distinguishable features of interface 76 are similar to what is shown in FIG. 13.

FIGS. 19 through 21 illustrate the cross-sectional views of intermediate stages in the formation of a multi-stacking package in accordance with alternative embodiments of the present disclosure. The initial steps of these embodiments are essentially the same as shown in FIGS. 1 through 9. Next, as shown in FIG. 19, a partial cut is performed to cut (for example, through sawing) into molding material 60, and hence forming trench 78 in molding material 60. The respective step is shown as step 220 in the process flow shown in FIG. 22. Trench 78 may be aligned to the center of the respective overlaying trench 66. The bottom of trench 78 may be at an intermediate level between the top surface of the top dielectric layer 46 and the bottom surface of the bottom dielectric layer 46. In accordance with some exemplary embodiments, width W4 of trench 78 is in the range between about 50 μm and about 80 μm. The depth of trench 78 may be in the range between about 50 μm and about 80 μm. Again, trenches 78 may form a grid, with dielectric layers 46 in the grid openings defined by the grid.

FIG. 20 illustrates the formation of molding material 74, for example, through applying and curing a liquid molding compound. Molding material 74 fills trench 78 (FIG. 19) and contacts the sidewalls and the top surface of molding material 60.

Package 70 is then de-bonded from carrier 30, and is sawed. The resulting package 72 sawed from package 70 is shown in FIG. 21. In the package 72 according to these embodiments, the portions of molding material 74 filled into trenches 78 (FIG. 19) form a ring encircling portions of molding material 60. Furthermore, molding material 74 also has a ring portion encircling dielectric layers 62 and at least some top portions of dielectric layers 46.

The embodiments of the present disclosure have some advantageous features. By patterning the polymer layers between lower-level dies and upper-level dies, the polymer layers are separated into discrete portions during the manufacturing process, and hence the stress caused to package by the polymer layers is reduced. The space that will otherwise be occupied by the polymer layers are hence occupied by molding compound, which includes fillers, and hence have a smaller effect in causing warpage in the package than large polymer areas.

In accordance with some embodiments of the present disclosure, a package includes a first molding material, a lower-level device die in the first molding material, a dielectric layer over the lower-level device die and the first molding material, and a plurality of redistribution lines extending into the first dielectric layer to electrically couple to the lower-level device die. The package further includes an upper-level device die over the dielectric layer, and a second molding material molding the upper-level device die therein. A bottom surface of a portion of the second molding material contacts a top surface of the first molding material.

In accordance with alternative embodiments of the present disclosure, a package includes a first molding material, a lower-level device die in the first molding material, a first polymer layer over the lower-level device die and the first molding material, and a first plurality of redistribution lines extending into the first polymer layer to electrically couple to the lower-level device die. An upper-level device die is disposed over the first polymer layer. A second molding material molds the upper-level device die therein, wherein a first edge of the first molding material and a second edge of the second molding material are in a same plane to form an edge of the package. A portion of the second molding material includes the second edge and a third edge opposite to each other, with the third edge contacting the first polymer layer. A through-via is disposed in the second molding material, wherein the through-via and one of the first plurality of redistribution lines electrically couple the lower-level device die to the upper-level device die.

In accordance with alternative embodiments of the present disclosure, a method includes molding a lower-level device die in a first molding material, planarizing the first molding material to expose the lower-level device die, forming a first polymer layer over the first molding material, patterning the first polymer layer to form a first trench, placing an upper-level device die over the first polymer layer, and molding the upper-level device die in a second molding material, wherein the second molding material fills the first trench to contact the first molding material. The second molding material is planarized to expose the upper-level device die.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method comprising: encapsulating a lower-level device die in a first encapsulating material; forming a first dielectric layer over the first encapsulating material; patterning the first dielectric layer to form a first trench; placing an upper-level device die over the first dielectric layer; encapsulating the upper-level device die in a second encapsulating material, wherein the second encapsulating material fills the first trench to contact the first encapsulating material; and planarizing the second encapsulating material.
 2. The method of claim 1, wherein the first trench fully encircles a portion of the first dielectric layer, with the portion of the first dielectric layer overlapping the lower-level device die.
 3. The method of claim 1 further comprising: forming a through-via over the first dielectric layer and electrically coupling to the lower-level device die, wherein during the encapsulating the upper-level device die, the through-via is encapsulated in the second encapsulating material, and after the planarizing, the through-via is exposed.
 4. The method of claim 1 further comprising: cutting the second encapsulating material to form a second trench in the second encapsulating material, wherein a bottom of the second trench is higher than a bottom surface of the second encapsulating material, and wherein the second trench forms a ring encircling the upper-level device die.
 5. The method of claim 4 further comprising filling a third encapsulating material into the second trench.
 6. The method of claim 1 further comprising performing a sawing step to form a discrete package comprising the lower-level device die and the upper-level device die, wherein a portion of the second encapsulating material filling the first trench remains in the discrete package.
 7. The method of claim 1, wherein the patterning the first dielectric layer to form the first trench simultaneously forms an opening in the first dielectric layer, and the method further comprises forming a redistribution line, wherein a portion of the redistribution line extends into the opening in the first dielectric layer to electrically couple to the lower-level device die.
 8. The method of claim 1 further comprising: forming a second dielectric layer over the second encapsulating material; patterning the second dielectric layer to form an additional opening and a second trench; and forming an additional redistribution line having a portion filling the additional opening, wherein the second trench is not filled when the additional opening is filled.
 9. The method of claim 8 further comprising sawing through the first trench and the second trench, wherein when the sawing is performed, the second trench is unfilled.
 10. The method of claim 8 further comprising: filling a third encapsulating material into the second trench; and sawing through the first trench and the third encapsulating material.
 11. A method comprising: encapsulating a first device die in a first encapsulating material; forming a first plurality of dielectric layers over the first device die and the first encapsulating material, wherein each of the first plurality of dielectric layers is formed by steps comprising: applying a dielectric layer; and patterning the dielectric layer to form a plurality of via openings and a first plurality of trenches in a common patterning process; forming a first plurality of redistribution lines having portions filling the plurality of via openings in each of the first plurality of dielectric layers, wherein the first plurality of trenches is not filled; and forming a plurality of electrical connectors electrically coupling to the first plurality of redistribution lines.
 12. The method of claim 11 further comprising sawing through the first encapsulating material, wherein kerves of the sawing pass through the first plurality of trenches that are unfilled.
 13. The method of claim 11 further comprising: performing a partial cut to form an additional trench extending into the first encapsulating material; and encapsulating bottom portions of the plurality of electrical connectors in a second encapsulating material, wherein the second encapsulating material comprises a lower portion filled into the additional trench.
 14. The method of claim 11 further comprising: encapsulating bottom portions of the plurality of electrical connectors in a second encapsulating material, wherein the second encapsulating material comprises a bottom surface substantially coplanar with a top surface of the first encapsulating material and a top surface of the first device die.
 15. The method of claim 11 further comprising: before the first device die is encapsulated in the first encapsulating material, forming a second plurality of dielectric layers, wherein the first device die is placed over the second plurality of dielectric layers; and forming a second plurality of redistribution lines in the second plurality of dielectric layers, wherein a second plurality of trenches is in the second plurality of dielectric layers after the second plurality of redistribution lines is formed, and the first encapsulating material is filled in the second plurality of trenches.
 16. A method comprising: encapsulating a first device die and a second device die in a first encapsulating material; planarizing the first encapsulating material to expose the first device die and the second device die; forming a first polymer layer over the first encapsulating material; patterning the first polymer layer to form a first trench and a first via opening simultaneously, wherein the first trench is directly over a portion of the first encapsulating material between the first device die and the second device die; forming a first redistribution line having a portion extending into the first via opening to contact the first device die, wherein the first trench is unfilled when the first via opening is filled; forming a second polymer layer over the first redistribution line and the first polymer layer; patterning the second polymer layer to form a second trench vertically aligned to the first trench; placing a third device die over the second polymer layer; and encapsulating the third device die in a second encapsulating material, wherein the second encapsulating material fills the first trench and the second trench.
 17. The method of claim 16 further comprising: patterning the second polymer layer to form an additional opening; and plating a metal post starting from the first redistribution line, wherein the metal post extends into the additional opening, and the metal post is encapsulated in the second encapsulating material.
 18. The method of claim 17 further comprising planarizing the second encapsulating material to expose the third device die and the metal post.
 19. The method of claim 16 further comprising sawing through the first encapsulating material and the second encapsulating material, wherein a kerf of the sawing passes a middle portion of each of the first trench and the second trench.
 20. The method of claim 16, wherein the first encapsulating material further encapsulates a plurality of device dies, and when the first trench is formed, a plurality of trenches is formed simultaneously, and the first trench and the plurality of trenches in combination form a grid of trenches. 